Material screening includes the use of vibratory screening machines. Vibratory screening machines provide the capability to excite an installed screen such that materials placed upon the screen may be separated to a desired level. Oversized materials are separated from undersized materials. Over time, screens wear and require replacement. As such, screens are designed to be replaceable. See, e.g., U.S. Pat. Nos. 7,578,394 and 7,228,971.
Replacement screen assemblies are subjected to large vibratory forces and must be securely fastened to the vibratory screening machine. Replacement screens are often attached to a vibratory screening machine by tensioning members, compression members or clamping members.
The manufacture of screen assemblies typically includes: fabricating a screening material, often three layers of a woven wire mesh; fabricating an apertured backing plate; and bonding the screening material to the apertured backing plate. Critical to screening performance are: the size of the openings in the screening surface and apertured plate; structural stability and durability of the screening surface; and structural stability of the entire unit. Drawbacks to conventional assemblies include lack of structure stability and durability of the screening surface formed by the woven wire mesh layers and lack of open screening area. These drawbacks limit the application and performance of the screen assemblies.
Weave patterns in metal woven screens, or cloths, include warp and weft wires woven together so that the warp wires are at an approximate 90 degree angle to the weft wires. The spaces between the wires form relatively small screening openings. Existing perforated plates, or apertured plates, typically have relatively larger perforations (when compared to the small screen openings of the woven screens) over which the woven screen spans. The size of the screen openings and plate apertures directly affect the open screening area of the entire screen assembly and its performance. The woven screens are attached to the perforated plates such that the edges of the perforations in the plate are perpendicular and parallel to the wires in the woven screen, see, e.g., FIG. 4 of U.S. Pat. No. 7,578,394. When subjected to the loading conditions of a vibratory screening machine and screen surface use conditions encountered during separation of materials (e.g., the weight of the material being screened on the surface of the screen assembly) the woven screens often fail along the line where the edge of a perforation of the perforated plate is aligned with the weave of the woven screen. Failures are frequently due to the parallel and perpendicular alignment of the woven screen wires and edges of perforations. Also, the distance the wires of the woven screens span the perforations in the plate directly impacts failures. Generally, the larger the distance the wires have to span the larger the deflection of the wire screen over the aperture and the larger the shearing force applied at the edges of the aperture.